This invention relates to a bushing in which the concentration of an electric field is moderated.
FIG. 1 shows a prior-art bushing. Referring to the figure, numeral 1 designates a central conductor, numeral 2 an upper terminal which is connected with the central conductor 1, and numeral 3 a porcelain tube which has the central conductor 1 inserted in the hollow part thereof. Symbol 3a denotes a fixture which is fastened to one end of the porcelain tube 3, while symbol 3b denotes a fixture which is fastened to the other end of the porcelain tube 3 and through which the upper terminal 2 and the other end of the porcelain tube 3 are hermetically fixed. Numeral 4 indicates an electrically grounded fitting flange on which the fixture 3a is mounted, and numeral 5 an electrode which is disposed in the bore part or annular hole of the fitting flange 4 and which moderates the concentration of an electric field at the upper end part of the fixture 3a. The electrode 5 is constructed in the shape of a cylinder encircling the central conductor 1 at a predetermined spacing therefrom, and it has a predetermined length from the one end toward the other end of the porcelain tube 3. A vessel 6 has a base portion 6a at the top end thereof, and the fitting flange 4 is mounted on the base portion 6a. Upon attaching the flange 4 to the base portion 6a, the one end of the porcelain tube 3 is hermetically fixed to the vessel 6 for receiving electrical equipment therein, and one end of the electrode 5 is connected with the vessel 6. The interior of the porcelain tube 3 as well as the vessel 6 is filled with an insulating gas 7 under a predetermined high pressure.
In a bushing of such construction, when a voltage is applied to the central conductor 1, equipotential surfaces 8 are distributed as indicated by broken lines in FIG. 2, and a concentration of the electric field lines develops at the upper end part A of the electrode 5.
Here, the upper end part A of the electrode 5 is surrounded with the high pressure insulating gas 7 exhibiting a high dielectric strength, and thus a high, corona causing, electric field strength develops therein. In contrast, the upper end part B of the fixture 3a is surrounded with atmospheric air, and hence, a low corona causing electric field strength is developed therein. When the height of the electrode 5 is reduced, the distribution of the equipotential surfaces changes to lower the electric field strength at part A and to raise that at part B.
In general, the electrical design of a bushing conforms with a procedure wherein, with reference to the highest voltage to be applied between the central conductor 1 and the fitting flange 4, the height of the electrode 5 is determined so that the electric field strength of part B becomes less than the breakdown voltage of the atmospheric air, while the electric field strength of the upper end part A of the electrode 5 is set to be less than the breakdown voltage of the ambient insulating medium. Accordingly, the flashover voltage of the assembled bushing is determined by the magnitude of the electric field strength of the upper end part B of the fixture 3a. As the upper end of the electrode 5 becomes higher than that of the fixture 3a, the electric field strength of part B lowers more, and the flashover voltage of the exterior rises more. However, the electric field strength of the upper end part A of the electrode 5 rises more as this part becomes more remote from the upper end part B of the fixture 3a. It is considered that, when parts A and B are made extremely distant, there will be a limit at which the conona generating voltage of part A becomes lower than that of part B and beyond which the flashover takes place between the upper end of the electrode 5 and the central conductor 1 inside the bushing.
For this reason, the bushing is so designed as to keep the height of the electrode 5 small in order that the breakdown may occur earlier from the upper end part B of the fixture 3a. In order to enhance the ability of the whole bushing to withstand high voltages, it has been necessary to increase the full length of the porcelain tube. This has led to the disadvantages that the height of the bushing must be greater than in the case of a condenser bushing, and that the cost of manufacture is increased.